Relativistic Euler's 3-body problem, optical geometry and the golden ratio

TL;DR

This paper investigates the properties of photon orbits in a two-black-hole Weyl solution, revealing a connection to the golden ratio and analyzing how these orbits depend on the black holes' separation.

Contribution

It introduces a detailed analysis of photon orbits in a symmetric two-black-hole spacetime, highlighting the emergence of the golden ratio in the merging of photon orbits.

Findings

Photon orbits merge at the golden ratio distance

No circular photon orbits beyond a critical separation

Optical geometry explains forbidden regions for time-like orbits

Abstract

A Weyl solution describing two Schwarzschild black holes is considered. We focus on the Z_2 invariant solution, with ADM mass M_{ADM}=2M_K, where M_K is the Komar mass of each black hole. For this solution the set of fixed points of the discrete symmetry is a totally geodesic sub-manifold. The existence and radii of circular photon orbits in this sub-manifold are studied, as functions of the distance 2L between the two black holes. For L-> 0 there are two such orbits, corresponding to r=3M_{ADM} and r=2M_{ADM} in Schwarzschild coordinates. As the distance increases, it is shown that the two photon orbits approach one another and merge when M_K=\varphi L, where \varphi is the golden ratio. Beyond this distance there exist no circular photon orbits. The two null orbits delimit a forbidden band for time-like circular orbits, which is interpreted in terms of optical geometry. For large L, time-like circular orbits are allowed everywhere, as in the analogous Newtonian problem. The analysis is generalised by considering a Z_2 invariant Weyl solution with an array of N black holes and also by charging the black holes, which connects the Weyl solution to a Majumdar-Papapetrou spacetime.